As electronics evolve, there is an increased need for miniature switches and microactuators that are provided on semiconductor substrates along with other semiconductor components to form various types of circuits. These miniature switches often act as relays, and are generally referred to as micro-electro-mechanical system (MEMS) switches. In many applications, MEMS switches may replace field effect transistors (FETs), and are configured as switches to reduce insertion losses due to added resistance, to reduce parasitic capacitance and inductance, and to reduce signal distortion inherent in providing FET switches in a signal path. MEMS switches are currently being considered in many radio frequency (RF) applications, such as antenna switches, mode switches, transmit/receive switches, switches in tunable networks, and the like.
Turning to FIGS. 1A and 1B, a MEMS device 10 having a MEMS switch 12 is illustrated according to one embodiment of the present disclosure. The MEMS switch 12 is formed on an appropriate substrate 14. The MEMS switch 12 includes a movable member, such as a cantilever 16, which is formed from a conductive material, such as gold. The cantilever 16 has a first end and a second end. The first end is coupled to the substrate 14 by an anchor 18. The first end of the cantilever 16 is also electrically coupled to a first conductive pad 20 at or near the point where the cantilever 16 is anchored to the semiconductor substrate 14. Notably, the first conductive pad 20 may play a role in anchoring the first end of the cantilever 16 to the semiconductor substrate 14 as depicted.
The second end of the cantilever 16 forms or is provided with a cantilever contact 22, which is suspended over a contact portion 24 of a second conductive pad 26. Thus, when the MEMS switch 12 is actuated, the cantilever 16 moves the cantilever contact 22 into electrical contact with the contact portion 24 of the second conductive pad 26 to electrically connect the first conductive pad 20 to the second conductive pad 26. The MEMS switch 12 may be encapsulated by one or more encapsulating layers 30, which form a substantially hermetically sealed cavity about the cantilever 16. The cavity is generally filled with an inert gas and may be sealed in a near vacuum state. Once the encapsulation layers 30 are in place, an overmold material 32 may be provided over the encapsulation layers 30 as part of a high volume packaging process. It is to be understood that a movable member having a fixed end and a free end such as cantilever 16 is but one type of configuration for a microactuator. Various types of microactuators known to those skilled in the art can have movable members that are fixed at more than one point or end.
To actuate the MEMS switch 12, and in particular to cause the cantilever 16 to move the cantilever contact 22 into contact with the contact portion 24 of the second conductive pad 26, an actuator plate 28 is disposed over a portion of the substrate 14 and under the middle portion of the cantilever 16. To actuate the MEMS switch 12, an electrostatic voltage is applied to the actuator plate 28. The presence of the electrostatic voltage over time creates a force that moves the metallic cantilever 16 toward the actuator plate 28, thus moving the cantilever 16 from the position illustrated in FIG. 1A to the position illustrated in FIG. 1B.
Problematically, actuation of a MEMS switch 12, especially one maintained at near vacuum conditions, results in the cantilever 16 moving toward contact portion 24 with a momentum sufficient to cause the cantilever contact 22 to bounce one or more times off of the contact portion 24 of the second conductive pad 26 after initial contact. Such bouncing degrades circuit performance and effectively increases the closing time. The article entitled “A Dynamic Model, Including Contact Bounce, of an Electrostatically Actuated Microswitch,” by Brian McCarthy et al., provides a detailed analysis of this bouncing phenomenon and is incorporated herein by reference. The dynamic closing forces may also be sufficient to damage both the contact portion 24 of the second conductive pad 26 as well as the cantilever contact 22, thus causing excessive wear, which results in a shortened operating life for the MEMS switch 12.
As a result, efforts have been made to control the speed with which the cantilever 16 moves cantilever contact 22 toward the contact portion 24 of the second conductive pad 26 at the moment of contact to reduce bouncing. In particular, an actuation signal having a special waveform is initially applied to the actuator plate 28. The actuation signal moves the cantilever 16 downward, such that the contact pad 22 at the end of the cantilever 16 initially moves rapidly toward the contact portion 24 of the second conductive pad 26. The actuation signal is configured such that the effective electrostatic voltage is reduced or removed prior to the cantilever contact 22 coming into contact with the contact portion 24 of the second conductive pad 26. The downward momentum will continue to move the cantilever 16 downward, albeit at a decreasing rate, wherein the contact pad 22 lands softly and slowly on the contact portion 24 of the second conductive pad 26. Once the MEMS switch 12 is closed, a hold signal is applied to actuator plate 28 to hold the cantilever 16 in a closed position such that the contact pad 22 is held in contact with the contact portion 24 of the second conductive pad 26. The article “A Soft-Landing Waveform for Actuation of a Single-Pole Single-Throw Ohmic RF MEMS Switch,” by David A. Czaplewski et al., provides a technique for providing a pre-determined actuation signal to control the closing of a MEMS switch 12 and is incorporated herein by reference.
Providing an actuation signal to effect soft closings of the MEMS switches 12 theoretically reduces bouncing and increases the operating life of the device. In practice however, process variation in the switch manufacture will reduce or eliminate the efficiency of a waveform to effect soft closing as described. Circuits which can adapt to this process variation in the switch manufacture have been proposed but these represent additional cost and complexity. Accordingly, there is a need for a technique to reduce or eliminate bouncing in MEMS switches 12 over process variations and operating conditions using simple waveforms and circuits.
MEMS switches 12 also have issues associated with being released from a closed position, or opening. The cantilever 16 can be effectively a metallic beam, which is deflected when the MEMS switch 12 is closed and is suspended in a natural state when the MEMS switch 12 is open. Releasing the MEMS switch 16 entails turning off the hold signal, and thus releasing the deflected cantilever 16 from the closed position. Once released the cantilever 16 springs upward and begins mechanically oscillating up and down. Such mechanical oscillation is referred to as ringing, and in a cavity in a near vacuum state this ringing may continue for an extended period of time. Such ringing may degrade circuit performance and effectively increases the opening time. Further, the magnitude and time of ringing may vary over various operating conditions and process variations.
If the cantilever 16 is still ringing when the next actuation signal is applied, the nominal actuation signal may not provide a bounce free closing given the cantilever's position, upward momentum, downward momentum, or a combination thereof. Accordingly, there is a further need for a technique to reduce or eliminate ringing of MEMS switches 12 over various operating conditions and process variations using simple waveforms and circuits. More generally, there is a need for a technique to suppress bouncing and ringing of movable members of MEMS microactuators that include MEMS switches, but also include MEMS capacitors and microscopic servomechanisms.
Summary of the Disclosure
The present disclosure provides a system and method for controlling positioning of a movable member of a micro-electro-mechanical system (MEMS) microactuator to reduce bouncing and ringing. The system includes a MEMS microactuator having a movable member relative to a fixed member, which is fixed to a substrate on which the MEMS microactuator resides. For example, in one embodiment, the MEMS microactuator is a MEMS switch, and the movable member is a cantilever. The system also includes control circuitry in communication with the MEMS microactuator, wherein the control circuitry is adapted to execute a method that increases an actuation signal from a first state to a second state to urge the movable member from a first position to a second position and hold the movable member in the second position. A rate of increase in the actuation signal is a positive slope with the lowest possible magnitude which will ensure full compliance with the predetermined application specification in the time allotted by that specification as the movable member moves from a first position to the second position. Using the lowest possible slope magnitude will ensure that the closing occurs with the smallest possible momentum and therefore will create the least possible amount of bouncing when using the simple positive slope waveform. For the purpose of this disclosure slope is defined as rise over run,
At some point, it is desirable to return the movable member to the first position. Therefore, the control circuitry is further adapted to decrease the actuation signal from the second state to the first state to release the movable member to the first position. In at least one embodiment, the rate of decrease for the actuation signal between the second state and the first state has a slope with a transition time that is not less than the inverse of one quarter of a natural frequency of the member in the first position as the movable member transitions from the second position to the first position.
In another embodiment, the control circuitry is further adapted to decrease the actuation signal from the second state to a third state to release the movable member to a third position and decrease the actuation signal from the third state to the first state to release the movable member to the first position. In this embodiment, the rate of decrease for the actuation signal from the second state to the third state can be as fast as or not less than the inverse of a quarter of a fixed-fixed natural mechanical frequency in which the both ends of the movable member are fixed. An example of a fixed-fixed movable member is a closed MEMS switch in which one end of the movable member is anchored and the other end has a contact that is firmly closed against a contact plate. In at least one embodiment, the rate of decrease for the actuation signal between the third state and the first state has a slope with a transition time that is not less than the inverse of one quarter of a natural frequency of the movable member in the first position as the movable member transitions from the from the third position to the first position.
In yet another embodiment, the control circuitry is further adapted to hold the movable member in the third position before decreasing the actuation signal between the third state and the first state. In this way energy stored in the movable member is further dissipated before proceeding from the third position to the first position.
Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.